A wavelength-division multiplexing (WDM) technique is one that combines (multiplexes) signals at different wavelengths to travel through the same optical fiber. The resulting signal is called a WDM signal and each wavelength of the WDM signal is called a wavelength channel (hereinafter “channel”). A WDM communication system is one that employs the WDM technique. For example, an optical communication system that comprises two nodes—one equipped with a wavelength-division multiplexer and the other, a demultiplexer—is a WDM communication system. Generally, the wavelength-division multiplexer multiplexes several single-wavelength signals into a WDM signal for transmitting through a single optical fiber to the demultiplexer, which demultiplexs the WDM signal back to several single-wavelength signals. However, a more complicated WDM communication system may provide blocking and/or add/drop capability in its nodes. The device that provides the blocking capability is called a wavelength blocker and enables a node to remove signals in some or all channels. The device that provides the add/drop capability is called a wavelength add/drop multiplexer (WAD) and allows a node to drop signals to other nodes or end users, add signals from other nodes or end users, or pass some or all input signals to the next node. Wavelength-division multiplexers and demultiplexers, WADs and wavelength blockers are examples of wavelength filters that “filter” signals in channels of a WDM signal.
As the demand for optical bandwidth increases in a WDM communication system, it is desirable to increase the number of channels. Unfortunately, this increases the size, the cost, and the insertion loss of these wavelength filters.